1. Field of the Invention
This invention relates generally to vibration-resistant fasteners, and more particularly to devices having cooperable thread formations especially adapted to resist inadvertent loosening from vibration once they have been assembled to one another.
2. Description of the Related Art Including Information Disclosed Under 37 CFR .sctn..sctn.1.97-1.99
The present invention has special utility as applied to the retention of a knob having a threaded stud to an ignitor plug body of an automotive-type electric cigar lighter.
A typical lighter is illustrated and described in U.S. Pat. No. 3,870,857 issued Mar. 11, 1975 to L. G. Horwitt and D. J. Mattis, and entitled CIGAR LIGHTER IGNITING UNIT. The ignitor plug comprises a tubular body in the form of a spacer member 46 and a transverse apertured end cap 48. The edges of the aperture form a partial thread which receives a stud secured to the lighter knob, by which the ignitor plug body can be grasped and held in the hand of the user. A pressure tooth 58 extends into the path of the stud, and the threads thereof experience a frictional force applied by the tooth, in order to improve the retention of the knob on the plug body.
The disclosed arrangement has been successfully employed over a period of many years and in thousands of cigar lighter units. It has been determined, however, that under certain circumstances there exists a tendency for the knob (and stud) to loosen from the plug body. This occurrence is undoubtedly a result of vibration associated with handling of the device during its final assembly, shipping, and subsequent installation in the dashboard of a vehicle.
Efforts to solve the problem by increasing the stiffness of the pressure tooth have been impractical, since the cap and tooth are constituted as a stamping from a relatively thin piece of sheet metal which would also have to be thickened if this approach were to be adopted. Similarly, the amount of torque than can be safely applied to the knob during assembly is limited, since care must be exercised to insure that the single turn thread formed by the edges of the aperture do not become "stripped" or otherwise deformed, by overtightening.